Catch Force Links and the Low to High Force Transition of Myosin

Abstract

Catch is characterized by maintenance of force with very low energy utilization in some invertebrate muscles. Catch is regulated by phosphorylation of the mini-titin, twitchin, and a catch component of force exists at all [Ca2+] except those resulting in maximum force. The mechanism responsible for catch force was characterized by determining how the effects of agents that inhibit the low to high force transition of the myosin cross-bridge (inorganic phosphate, butanedione monoxime, trifluoperazine, and blebbistatin) are modified by twitchin phosphorylation and [Ca2+]. In permeabilized anterior byssus retractor muscles from Mytilus edulis, catch force was identified as being sensitive to twitchin phosphorylation, whereas noncatch force was insensitive. In all cases, inhibition of the low to high force transition caused an increase in catch force. The same relationship exists between catch force and noncatch force whether force is varied by changes in [Ca2+] and/or agents that inhibit cross-bridge force production. This suggests that myosin in the high force state detaches catch force maintaining structures, whereas myosin in the low force state promotes their formation. It is unlikely that the catch structure is the myosin cross-bridge; rather, it appears that myosin interacts with the structure, most likely twitchin, and regulates its attachment and detachment.

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…Since twitchin is a thick filament protein, this would bind the two filaments and provide the necessary rigidity, and the phosphorylation dependence of the interaction explains how catch is controlled. However, this still left a nagging problem: although the actomyosin crossbridges were not the stress resistant structure in the catch state, various lines of evidence showed that when myosin is generating force catch force decreases, and when it is not, catch force increases, suggesting that the twitchin linkages and actomyosin force generating crossbridges interact in some way (Butler et al, 2006).…”

“…Thick filament is bottom thick line, the myosin head is the object resembling a microphone, the thin filament is the row of open circles, and twitchin is the object represented as a coil (unphosphorylated and unable to interact with the thin filament) and straight lines (interacting with the thin filament in alteration with myosin in ACh and continuously during catch in saline). Modified from Funabara et al (2005) and Butler et al (2006).…”

The N-terminal Region of Twitchin Binds Thick and Thin Contractile Filaments

“…Determination of IGDXD1IG and DXD1 binding to thin filaments was done using a modification of the co-sedimentation technique described by Shaffer et al (32) for measuring binding of various fragments of myosin-binding protein C to F-actin. The assay solution was based on a relaxing solution used for permeabilized muscles (24,25). It contained 30 mM PIPES, 20 mM EGTA, 3 mM MgATP, 3 mM free Mg 2ϩ , 1 mM DTT, and 21 mM 1,6-diaminohexane-N,N,NЈ,NЈ-tetraacetic acid (HDTA), pH 6.8.…”

“…Mechanical Measurements on Permeabilized ABRM-Muscles were dissected, permeabilized with Triton X-100, and subjected to mechanical measurements as described previously (24). The relaxing solution contained 3 mM MgATP, 5 mM phosphocreatine, 20 mM EGTA, 3 mM free Mg 2ϩ , 1 mM DTT, 30 mM PIPES, 15.6 mM HDTA, 1 mg/ml creatine phosphokinase, 0.5 mM leupeptin, pH 6.8.…”

“…Their results supported our previous suggestion (22) that twitchin might provide a mechanical link between thick and thin filaments. The idea that catch force is maintained by a mechanical link not involving the high force myosin cross-bridge is also supported by studies showing that agents that block force output by myosin cross-bridges have no effect when added to permeabilized muscles in catch (23,24) and actually increase catch force when added to activated muscles (24). Also, there is no measurable turnover of the ADP bound to myosin when catch force is relaxed by phosphorylation of twitchin (25).…”